1. Field of the Invention
Embodiments of the invention are generally directed to the field of fiber optic communication and more particularly to a method and apparatus for mitigating the effects of modal dispersion in multimode optical fiber and fiber optic communication systems incorporating these methods and apparatus.
2. Description of Related Art
It is known to propagate optical communication signals over the medium known as optical fiber. The most general types of optical fiber are referred to as single mode fiber and multimode fiber. Optical fiber generally consists of two parts: a core region and a cladding region surrounding the core region. The core region and the cladding region have different optical characteristics, most notably the core region has an index of refraction, n1, that is greater than the index of refraction, n2, of the cladding region. A single mode fiber typically has an outer diameter of approximately 125 microns and a core diameter on the order of 2–10 microns. In contrast, a multimode glass fiber having an outer diameter of approximately 125 microns may typically have a core region diameter on the order of 50–65 microns (50 and 62.5μ being typical multimode core diameter values). The difference between the optical characteristics of the core region and the cladding region allow light to be guided in the core region of the fiber and substantially confined to the core region without leaking into the cladding region.
In consideration of factors including fiber geometry and optical parameters such as index of refraction, various conditions are established that control the characteristics of the light propagating along the fiber. These characteristics are referred to as fiber modes. A mode can basically be thought of as a self-repeating intensity distribution. An acoustic mode, for example, is well illustrated by the standing wave pattern of a guitar string at a particular frequency or sound. Different frequencies (different sounds) produce different patterns of string vibration that represent different modes of the sound waves. Similarly, in the optical realm, light of different colors is defined by the wavelength (or frequency) of the light. Other discernable characteristics of light wave transmission are its polarization and the strength of the electric and magnetic fields of the light. These characteristics define the various possible patterns of the light referred to as modes. Illustrations of several transverse Gaussian laser mode patterns having cylindrical symmetry (TEMpl) are shown in FIGS. 1(a–l), where p, l are integers labeling the radial and angular mode order. Similar transverse Gaussian laser mode patterns having rectangular symmetry (TEMmn) are shown in FIGS. 2(a–l), where m, n are integers representing the horizontal and vertical mode orders. The lowest order mode, denoted as the (0,0) mode in FIGS. 1a, 2a, resembles a substantially uniform intensity circle of light. FIG. 2b illustrates a (1,0) mode that is characterized by a vertical division creating two lobes as shown. FIGS. 1(a–l) and 2(a–l) show mode patterns having various (p,l) and (m,n) values as shown.
As their names indicate, a single mode optical fiber restricts the propagation of light modes along the fiber to a single mode. This is typically the (0,0) mode as it is the most stable. In a similar manner, a multimode fiber supports the simultaneous propagation of multiple modes.
For reasons well known in the art, the various modes propagating in a length of multimode fiber travel along the fiber at different velocities. The different velocities of these modes gives rise to the phenomenon of dispersion. Dispersion, in optical terms, refers to the spreading or separation of a wave into different spectral components as seen, for example, by the effects of a prism on white light. In a waveguide, such as a multi-mode optical fiber, the dispersion of the modal patterns of light in the fiber is referred to as modal dispersion, (sometimes referred to as multimode distortion). Modal dispersion is the temporal spreading (distortion) of the optical signal (light pulses) due to the different propagation velocities of the different modes. This temporal pulse distortion, or modal dispersion, results in what is known as intersymbol interference (ISI) at the receiver or signal detector. A diagram of ISI is illustrated in FIG. 3 and can be understood as follows: In optical fiber communications, the signal information is coded into trains of optical pulses. These information pulses, contained in the propagating modes, arrive at a distant end point (e.g., receiver) of the system at different times due to the effect of modal dispersion; i.e., an original pulse of data at the input end of the fiber spreads apart into several pulses at the receiver end of the fiber. Intersymbol interference caused by the modal dispersion is the principal limiting factor affecting what is known as the bit rate-distance product of the fiber communication system. The bit rate-distance product is a metric that measures the information capacity of the optical fiber. It is well known that the ISI at the receiver is a direct function of the modes that are excited or launched at the input end of the fiber. For example, strongly focused light at the input will excite a large number of modes due to the strong convergence angle of the light. Each mode will travel with a group (envelope) velocity, (vg)m,n. If all the modes from (0,0) to (mmax, nmax) are excited, the output pulse at a distance, z=L, will broaden to a value expressible asΔτ≅L[(1/(vg)m max, n max)−(1/(vg)0,0)].It can be shown that the maximum number of pulses per second that can be transmitted without serious overlap of adjacent output pulses isfmax˜1/Δτ.Thus, high data rate transmission is maximized by single mode excitation, and single mode propagation through single mode optical fiber. (See, Yariv, Introduction to Optical Electronics, Second Edition, Holt, Reinhart & Winston, NY, Cha. 3 (1976)), incorporated herein in its entirety to the fullest allowable extent. However, many of the existing legacy fiber installations found, for example, on ships, in aircraft and throughout local area networks consist of multimode, premises optical fiber that cannot be replaced by single mode fiber in a cost effective manner. Accordingly, solutions are required that cost effectively improve the bit rate-distance product capacity of these multimode installations.
Several techniques have been developed and deployed to control the modes that are excited and launched in multi-mode optical fiber. One technique relies on the use of spatial light modulators to perform adaptive spatial filtering such that only the desired modes are excited. The interested reader is directed to Alon et al., EQUALIZATION OF MODAL DISPERSION IN MULTIMODE FIBER USING SPATIAL LIGHT MODULATORS, Department of Electrical Engineering, Stanford University, CA 94305, the subject matter of which is herein incorporated by reference in its entirety to the fullest allowable extent. Alternatively, a technique known as radial offset launch has been demonstrated to selectively launch advantageous modes. In this side launch technique, the input light to the system is launched off-center and off-angle with respect to the core of the multimode fiber. However, both of the above mentioned techniques for mitigating modal dispersion in multimode fiber and reducing the resulting degradation of the bit rate-distance product due to ISI are costly and technically complex.
Accordingly, the inventors have recognized a need for a more cost-effective, less complex approach to the mitigation of modal dispersion in multimode fiber that additionally is less sensitive to radial fiber perturbations, and thus more immune to modal dispersion induced ISI. Further, an approach is sought that will be less impacted by the defect-ridden core of many legacy multimode fibers, which mitigates the effects of deterministic jitter by mode launch conditions coupled with modal dispersion within the fiber, and which provides other benefits and advantages over currently known solutions as will be recognized by those skilled in the art.